The invention concerns electret-enhanced filter media (more simply called xe2x80x9celectret filtersxe2x80x9d) made of fibers such as melt-blown microfibers. The invention concerns an improved method for making fibrous electret filters for removing particulate matter from air. The invention is especially concerned with respirators and improving the level of filtration-enhancing electrostatic charges on the filter media.
For many years nonwoven fibrous filter webs have been made from polypropylene using melt-blowing apparatus of the type described in Report No. 4364 of the Naval Research Laboratories, published May 25, 1954, entitled xe2x80x9cManufacture of Super Fine Organic Fibersxe2x80x9d by Van A. Wente et al. Such melt-blown microfiber webs continue to be in widespread use for filtering particulate contaminants, e.g., as face masks and as water filters, and for other purposes, e.g., as a sorbent web to remove oil from water, acoustic insulation and thermal insulation.
The filtration quality of a melt-blown microfiber web can be improved by a factor of two or more when the melt-blown fibers are bombarded as they issue from the die orifices with electrically charged particles such as electrons or ions, thus making the fibrous web an electret. Similarly, the web can be made an electret by exposure to a corona after it is collected. Melt-blown polypropylene microfibers are especially useful, while other polymers may also be used such as polycarbonates and polyhalocarbons that may be melt-blown and have appropriate volume-resistivities under expected environmental conditions.
Fibrous filters for removing particulate contaminants from the air are also made from fibrillated polypropylene films. Electret filtration enhancement can be provided by electrostatically charging the film before it is fibrillated.
Common polymers such as polyesters, polycarbonates, etc. can be treated to produce highly charged electrets but these charges are usually short-lived especially under humid conditions. The electret structures may be films or sheets which find applications as the electrostatic element in electro-acoustic devices such as microphones, headphones and speakers, in dust particle control, high voltage electrostatic generators, electrostatic recorders and other applications.
In one aspect, the present invention provides a method of charging a nonwoven web of thermoplastic microfibers to provide electret filter media comprising impinging on a nonwoven web of thermoplastic nonconductive microfibers capable of having a high quantity of trapped charge jets of water or a stream of water droplets at a pressure sufficient to provide the web with filtration enhancing electric charge and drying said web. Surprisingly, it has been found that merely by impinging these jets of water or stream of water droplets onto the nonwoven microfiber web, the web develops filtration enhancing electret charge. The charging can be further enhanced by subjecting the web to corona discharge treatment prior to impingement by the water. Preferably, the web is formed from melt blown polypropylene microfibers, poly(4-methyl-1-pentene) microfibers or blends thereof. The term xe2x80x9chydrochargingxe2x80x9d will be used herein to describe this method.
The webs appear to be charged after impingement by jets of water or a stream of water droplets because when a hydrocharged web is exposed to unfiltered x-ray radiation, the filtration efficiency is markedly reduced.
The fibrous electret filter produced by the method of the present invention is especially useful as an air filter element of a respirator such as a face mask or for such purposes as home and industrial air-conditioners, air cleaners, vacuum cleaners, medical air line filters, and air conditioning systems for vehicles and common equipment such as computers, computer disk drives and electronic equipment. In respirator uses, the electret filters may be in the form of molded or folded half-face masks, replaceable cartridges or canisters, or prefilters. In such uses, an air filter element produced by the method of the invention is surprisingly effective for removing particulate aerosols. When used as an air filter, such as in a respirator, the electret filter media has surprisingly better filtration performance than does a comparable electret filter charged by known methods.